HEAT T-REATMEHT. 285 more suitable for structural work than the ordinary product of a mill, while assuredly the extra cost of such careful treatment of long and heavy sections would make it commercially out of the question in almost all cases. It is, of course, understood that the treatment of eye-bars is a different question, this being made necessary by the work done in shaping the ends. TABLE XV-K Comparison of the Natural and Annealed Bars shown in Table XV-J, which show about the same Ultimate Strength. •AtSd •° S3 ** 0 <U Limits of 3 "rt p, ft o Ultimate Strength; Ibs. per sq. Kind of Steel. No. ol bars. Condition of bars. ^a* (D*-1 <a yy i S.S-H '|s . J... '•S 6 •rt inch, § 3 & ^ 5 o< O C S •6 ^ 3 3 .5 w' ° dj.O B- .541058,000 Acid. 10 Natural 56.200 39,550 29.7 58.8 70.4 52 to 59. 000 15 Annealed 55,690 36180 36 3 56.8 64.9 55 to 58,000 Basic. 12 Natural 56.8-0' 37,760 30.4 56.4 66.4 54 to 64,000 17 Annealed 57,870 35,320 36.6 57.6 61.0 55 to 60.000 Acid.* 4 Natural 58.130 40400 30.1 61.7 69.5 55 to 60,000 7 Annealed 65,021 31,576 30.4 60.0 57.4 SEC. XVh.f—General remarks on the determination of tempera* lures.—For the commercial operation of annealing, the temperature may be conveniently and accurately determined by the use of a platinum or copper ball with the usual water receiver. In more accurate work it is advisable to use a Le Chatelier pyrometer, but in either case considerable care must be taken to insure that the piece of metal which registers the temperature, whether it be the ball or the electric couple, is of the same degree of heat as the forgr ing or the casting under treatment. It is generally taken for granted that if the juncture of a Plati-•num—Platinum—ten per cent. Ehodium couple is in contact with the steel under treatment, the temperature as registered is correct. Practically, although not absolutely, this is true, for if the conditions of heating are the same, that is, if the furnaces are of the same general size and plan and the pieces under treatment are * These constitute Group III In Table XV-C. t The remainder of this chapter is mainly the work of J. W. Campbell.